said by DKS:
I remember one of those with a little vial of mercury to help tip the spring over. Probably a toxic hazard, now.
The little vial of mercury wasn't there to help tip anything over. Look closely at one and you'll see that there are also two wires inside the vial. The thermostat calls for HVAC "On" when the spring tilts the vial enough to move the mercury to the end of the vial where the two wires are located. Mercury is a metal and is conductive. The mercury completes the circuit by shorting the two wires to each other when it rolls to the "wired" end of the vial.